The present invention relates to reset circuits for providing a reset signal, and in particular to such a circuit incorporating a delay.
In electrical systems power interruptions are common. A sophisticated system contains power-on and low voltage detection delayed reset circuitry to provide a reset signal following the turn-on of power or a power interruption. The low voltage detection circuit resets a power-on delay timer, and thus upon power restoration the delay reset functions as though the power has just been turned on. The disadvantage in this approach is that a very short power interruption pulse causes a completely new reset cycle, causing the entire system to be reset, which can take a very long time depending on the system design.
In actual applications many loads have long L/R or RC time constants which cannot respond in the short time that the power interruption lasts, and there is no danger from a short interruption. However, once the reset is issued the loads are forced to go through the reset sequence. In some applications this institutes the entire reset process, which may be lengthy. For example, in systems where a microprocessor is used, the entire start up procedure needs to be reinitiated, which may require human interaction. An example that is very common is when a PC resets during a power surge. After such an incident one needs to restart all applications and in some cases redo extensive work if there was no recent save.
The prior art XR8000 chip contains a 5V regulator featuring power on delay, low voltage detect and a watchdog timer. The XR8000 issues a reset signal on power up and low voltage detect. Hence, since most power interrupts produce low voltage detection, a reset signal is issued upon every power interrupt.
In many cases, during a very short power interruption, the regulated supply voltage drops by a small amount, but it remains within the specified operating range for the microprocessor, which means that if the processor were to simply store its status as soon as the power interruption occurs and goes into standby mode for the duration of the interrupt, operating conditions can be restored as soon as the power is restored. If the power interrupt exceeds a predetermined time that causes the regulated supply to fall below the specified operating range, a normal reset could be issued.